ENHANCED RAMAN-SPECTROSCOPY AT A NONMETALLIC SURFACE .1. SPACER LAYERS OF ALKYL MERCAPTANS ON SILVER ISLAND FILMS

Self-assembled monolayers of alkyl mercaptans were used as spacer laye rs on silver island films. N-Alkylmercaptans (8, 12, and 16C) yielded low-energy, hydrophobic upper surfaces but carboxyl-terminated undecyl mercaptan gave a hydrophilic upper surface. Surface-enhanced Raman sc attering (SERS) was observed from two different molecules deposited on the alkyl mercaptans/island film system. One was p-nitrobenzoic acid (PNBA), a small molecule able to chemisorb on silver. The other, cobal t phthalocyanine (CoPC), was bigger and only physisorbed on the substr ates. For every spacer/scatterer pair, special care was taken of the e xact location of the molecule. The small molecule (PNBA) allowed chara cterization of the coverage of both types of spacer layers, hydrophobi c and hydrophilic. PNBA could always penetrate into the alkyl chains, whatever the spacer. Substrates with hydrophobic and hydrophilic upper surfaces gave very different spectra after deposition of the bigger m olecule (CoPC). When CoPC was located at about 15-20 Angstrom from sil ver, the SERS signal was about 10 times lower than when CoPC was adsor bed directly onto the metal. These results constitute a first measurem ent of the spatial extension of SERS with strong evidence that the sca ttering molecule was not on the metallic surface.